arm: detect NEON on FreeBSD armv6

FreeBSD 11.2 added AT_HWCAP via sysctl() while 12.0 added elf_aux_info()
which requires less code to use and is cached by libc. Only support the
latter to avoid cruft to be removed after 11.* EOL on 2021-09-30.

meson.build

@@ -147,6 +147,9 @@ if (host_machine.cpu_family() == 'aarch64' or
     if cc.has_function('getauxval', prefix : '#include <sys/auxv.h>', args : test_args)
         cdata.set('HAVE_GETAUXVAL', 1)
     endif
+    if cc.has_function('elf_aux_info', prefix : '#include <sys/auxv.h>', args : test_args)
+        cdata.set('HAVE_ELF_AUX_INFO', 1)
+    endif
 endif
 
 # Compiler flag tests

src/arm/cpu.c

@@ -37,6 +37,11 @@
 #endif
 #define NEON_HWCAP HWCAP_ARM_NEON
 
+#elif defined(HAVE_ELF_AUX_INFO) && ARCH_ARM
+#include <sys/auxv.h>
+
+#define NEON_HWCAP HWCAP_NEON
+
 #elif defined(__ANDROID__)
 #include <stdio.h>
 #include <string.h>
@@ -77,6 +82,10 @@ unsigned dav1d_get_cpu_flags_arm(void) {
 #elif defined(HAVE_GETAUXVAL) && ARCH_ARM
     unsigned long hw_cap = getauxval(AT_HWCAP);
     flags |= (hw_cap & NEON_HWCAP) ? DAV1D_ARM_CPU_FLAG_NEON : 0;
+#elif defined(HAVE_ELF_AUX_INFO) && ARCH_ARM
+    unsigned long hw_cap = 0;
+    elf_aux_info(AT_HWCAP, &hw_cap, sizeof(hw_cap));
+    flags |= (hw_cap & NEON_HWCAP) ? DAV1D_ARM_CPU_FLAG_NEON : 0;
 #elif defined(__ANDROID__)
     flags |= parse_proc_cpuinfo("neon") ? DAV1D_ARM_CPU_FLAG_NEON : 0;
 #elif defined(__APPLE__)